A semiconductor integrated circuit chip is typically fabricated with a back-end-of-line (BEOL) interconnect structure, which comprises multiple levels of metal wiring and inter-level metal vias, to connect various integrated circuit components and devices that are fabricated as part of a front-end-of-line (FEOL) layer of the semiconductor integrated circuit chip. Current state of the art BEOL process technologies typically implement copper to form BEOL interconnects, as the use of copper interconnects, as compared to conventional aluminum interconnects, are known to significantly reduce resistance in the BEOL interconnect structure, resulting in improved conduction and higher performance. As copper interconnect structures are scaled down, however, current density in the copper interconnect structures increases exponentially, resulting in current-driven electromigration of copper atoms. In the context of copper interconnect structures, electromigration is the transport of the copper material caused by the gradual movement of ions in the copper interconnect structures due to momentum transfer between conducting electrons and diffusing copper atoms. When copper interconnect structures undergo electromigration, the copper atoms can diffuse away from the copper interconnect structures and into other regions of the integrated circuit, thereby leading to various defects in the copper interconnect structure such as, for example, voids and hillock defects.
To mitigate electromigration in BEOL copper interconnect structures, a capping layer is typically formed over the BEOL copper wiring. The capping layer is formed with a material that serves to inhibit diffusion, oxidation, and/or electromigration from the top surface of the copper interconnect structures and/or to prevent oxidation of the copper interconnect structures. When forming a capping layer on a copper interconnect structure, however, the quality of the interface between the capping layer and the copper interconnect structure is extremely critical for purposes of preventing electromigration, because if there are too many impurities such as oxygen atoms, carbon atoms, etc. at the interface, there can be electromigration and unwanted reactions between the copper interconnect structure and the capping layer.